Catalytic reforming, using Pt based reforming catalyst, is one of the most important refinery processes in the world. Most refineries have a catalytic reformer, which converts naphtha fractions into high octane reformate.
Reformers come in many types and sizes--from 2000 BPD fixed bed units to moving or swing bed units processing more than 50,000 BPD. Reformers are available with fixed bed reactors, swing bed reactors, or moving bed reactors. Many new units are moving bed reactors, available from UOP, Inc, Des Plaines, Ill.
Reformers generally use mono-metallic catalysts (Pt on a support such as alumina) or hi-metallic catalyst (Pt-Re on a support). Other combinations of Pt and other metals are known. All reforming catalyst are believed to contain a halogen, almost invariably chlorine. The presence of chlorine is beneficial for the reforming process, and may be essential for successful regeneration of Pt catalyst, as the Cl helps keep the Pt dispersed as small crystals on the catalyst.
While all reformers are believed to have some chloride compounds in the reformate, the problem is most serious when a continuous reformer is used, and especially so when the catalyst is near the end of its useful life.
Some refiners add chlorine compounds continuously to their units to maintain a high chloride level on the catalyst. In continuous or moving bed reformers the catalyst is chlorided after coke burn but before return to the top of the reforming reactor. More chlorine is added now, as opposed to 10 or 20 years ago, both as a prophylactic measure to allow the units to be pushed harder, and the belief that catalyst regeneration is more successful with more Cl on catalyst.
Cl in the reformate causes problems in downstream units. The main chloride compounds in reformate are believed to be HCl, NH.sub.4 Cl and FeCl.sub.3. Some refiners may use other halogens, such as Fl or I, but Cl is the halogen of choice, so hereafter chlorine and its reaction or degradation products will be referred to rather than halogens in general.
Chlorine compounds in reformate cause several problems. Some regions have a pH specification on gasoline, which can not be met if large amounts of HCl are present in the reformate. Chlorides can seriously affect downstream processing units, such as a Sulfolane aromatics extraction unit, if the reformate is so treated.
Chlorides can cause very immediate problems in the reformer. If the reformer is relatively dry, as most are, the chlorides form salts which plug up the reformer fractionators. If water is added to wash the salts out then HCl is formed, which causes serious corrosion problems. As an example, one of our refineries had a problem with chloride salt buildup in product fractionators. Every three months or so the fractionator efficiency declined so that it was necessary to water wash the column. About 1 wt % water was added to the tower to wash out salts. This cleaned the column, but would also form some HCl, which can attack some steels, especially with water present.
The problem has gotten worse in the last decade, going from nuisance to major problem. The conventional methods of handling chloride in reformate will be briefly reviewed. These are grouped arbitrarily below and reviewed in detail hereafter.
1. Water washing, PA1 2. Solid adsorbent treating of reformate, PA1 3. Chemical treatments.
1. Water Washing
Water washing of a depropanizer fractionating tower that was part of a continuous catalytic reformer was reported in Example 2 of U.S. Pat. No. 4,880,568. Periodic water washing for a severe fouling and corrosion problems was not effective, "an elaborate continuous water wash system was installed. The continuous water wash system also failed to solve the deposit problem." Such a system also introduces water into the process which water will cause additional problems.
Example 2 of '568 was directed to continuous or intermittent treatment of a chloride containing fraction of a reformate.
Somewhat related is an aqueous, alkaline treatment of the reformate liquid upstream of the debutanizer. We tried a brief test in one of our commercial refineries at solving a chloride problem by injecting dilute caustic into reformate intermediate the V/L separator and the debutanizer. The caustic was less than 15.degree.or 20.degree. C. A mesh pad was used to aid in separation of caustic/reformate in a separator vessel. The experiment was not considered a success. A flow control valve corroded, and the experiment was stopped.
Probably the contact between caustic and reformate was poor. The addition of water would have also caused problems.
2. Solid Adsorbent Treating
Some refiners use beds of solid adsorbent material to prevent chloride corrosion and fouling. More details about this type of treatment are available from UOP Inc which has endorsed use of at least one type of solid adsorbent to remove chlorides from reformate.
Such solid adsorbent beds can plug, and many refiners do not want to use that approach. Such adsorbents are also believed to be expensive, typically involving proprietary adsorbents. At least some of these proprietary materials are thought to be ineffective for removing NH.sub.4 Cl.
Somewhat related to the above solid bed treatment of reformate streams is the use of a somewhat porous, relatively densely poured bed of granular alkalies to treat a variety of hydrocarbon streams in Sun, U.S. Pat. No. 3,761,534, which is incorporated by reference.
Example 1 used 4-8 mesh granular NaOH to remove sulfuric acid from an alkylate stream of tert.--butylated ethyl-benzene containing about 0.3N total acid, primarily sulfuric acid. Although efficient acid removal first occurred, the bed plugged before 100 volumes of alkylate could flow through the bed.
Example 4 used no NaOH, but treated an effluent from the alkylation of benzene with ethylene in the presence of HCl with soda lime and glassmaker's (G. M.) alkali to remove acid. Example 5 used pellets of C. P. NaOH to treat crude tert. butylated ethyl-benzene containing 570 ppm H.sub.2 SO.sub.4. NaOH pellets plugged at 92 weights of alkylate per weight of alkali, while beds of soda lime and G. M. alkali did not plug.
Example 7 used G. M. alkali on a support grid to treat crude tert.butylated ethylbenzene containing about 600 ppm sulfuric acid. The organic flowed up through the support grid, through the alkali to an outlet above the bed of alkali. A white precipitate built up in the reservoir below the grid, which was periodically removed through a drain valve by a water purge. The bed of alkali was reported essentially unchanged by casual observation and there was no increase in resistance to flow through it.
The streams treated in '534 were probably saturated with water, and/or carried entrained water, as periodic water purges were reported in many examples. Some of the results reported could be summarized as follows:
Beds of caustic pellets do not work for very long to remove acidic contaminants from such liquid hydrocarbon streams.
All beds plug in downflow operation or rapidly lost effectiveness. Upflow operation with alkali on a support of a grid or coarse screen works a long time because salts that form can fall down through the screen.
Porous G. M. alkali was better than solid caustic.
3. Chemical Treatments
Several patents are directed at adding treatment chemicals which inhibit the formation of ammonium chloride in units, and are believed directed at keeping chloride compounds in a form which will not precipitate as a solid in process equipment. Some treatment programs include chelating agents and/or film forming agents to prevent further corrosion.
U.S. Pat. Nos. 5,282,956 and 5,256,276, which are incorporated by reference, disclose inhibiting ammonium chloride deposition by adding an amide such as 1,3-dimethyl-2-thiourea or phosphatide such as lecithin.
U.S. Pat. No. 2-thiourea 4,880,568, METHOD AND COMPOSITION FOR THE REMOVAL OF AMMONIUM SALT AND METAL COMPOUND DEPOSITS, Staley et al, Assignee Aqua Process, Inc., Houston, Tex. discloses injecting amines and chelating agents into reformate to remove and/or prevent formation of ammonium salt deposits. Amines added form amine salts with a low melting point or an affinity for trace amounts of water. This patent is incorporated by reference.
While adding chemicals to prevent formation of ammonium chloride deposits and/or chelating agents to remove metal corrosion products will help, such approaches are expensive and are not considered the ideal solution. Film forming agents may still be needed to protect metal surfaces in process equipment. Additives added will end up in one or more product streams, and these additives may cause additional problems downstream.
Many refiners would prefer to eliminate the problem, if possible, rather than add more chemicals to their reformate which must be dealt with in downstream processing units.
I studied the problem of chloride removal from reformate, and found nothing that was completely satisfactory.
The conventional approaches had several shortcomings. Unconstrained contact of reformate with dilute caustic was not successful in our refinery test. Continuous water washing was not successful in a depropanizer, as reported in U.S. Pat. No. 4,880,568.
I had concerns about adding more water to refinery streams. Catalytic reformate is a dry stream, passing through multiple distillation columns prior to reforming. Adding water to such a heretofore dry stream may (and has) cause corrosion or other problems in downstream units.
One of our refineries tried a proprietary method of dealing with chloride in reformate involving addition of chemicals, but the cure was worse than the disease.
I wanted to remove chlorides entirely from the reformate, not merely convert them to less noxious materials. I wanted to remove them, but without adding other chemicals to the reformate stream, and especially without adding a lot of water to the reformate.
I was concerned that solid adsorbent beds were likely to plug and difficult to regenerate. I knew that a liquid based system could be made to work, as disclosed in my earlier application, Ser. No. 08/217,821 filed on Mar. 25, 1994. There I disclosed a way to remove essentially all of the Cl from typical reformate streams using a water based reactive extraction process. While that process is a significant advance over the state of the art, it did have some disadvantages, which are reviewed below.
My earlier process used an aqueous solution to treat the reformate. This always added a minor amount of water to the reformate stream. Although the amount added could be much less than equilibrium, some refiners wish to keep their reformate streams as dry as possible. This meant that a liquid solution had to be prepared and perhaps stored. Some refiners were concerned that some of this aqueous solution might be entrained in the reformate. The process also produced a relatively dilute brine byproduct as a result of removing halogen from the liquid reformate stream.
I have now discovered a better way to remove halogens from reformate and similar naphtha hydrocarbon streams which does not require any aqueous reagents. I found that solid caustic can efficiently remove halogens from reformate in a completely dry system.
One key to making the process work was selecting a stream which was relatively dry for treating, or rather in applying this process only to selected streams which were not saturated with water. If this process is tried on water saturated streams, the solid caustic bed will soon plug, and the desired form of salt precipitation, discussed below, will not occur.
By treating dry streams, with non-porous solid caustics in a bed with a large interstitial volume, most of the salt that forms from the neutralization reaction can be deposited on the surface of the solid caustic. This salt can be safely held on the surface of the solid caustic as a relatively soft fluffy deposit. It looked much like rust on an iron plate. This salt could be held by the caustic and fill up interstitial places in the caustic bed, without plugging the bed.
Significant run lengths can be achieved when treating liquid hydrocarbon streams not saturated with water with a dry, solid caustic bed. This makes the process a worthy substitute for alumina treaters even without regeneration of the caustic. I also developed a caustic bed regeneration procedure, which can selectively dissolve such salt deposits, in preference to caustic, which multiplies the cost effectiveness of solid bed treating.